Ethylene is produced worldwide in large quantities, primarily for use as a chemical building block for other materials. Ethylene emerged as a large volume intermediate product in the 1940s when oil and chemical producing companies began separating ethylene from refinery waste gas or producing ethylene from ethane obtained from refinery byproduct streams and from natural gas.
Most ethylene is produced by thermal cracking of hydrocarbon with steam. The arrangement of a typical ethylene cracking furnace is shown in FIG. 1. Hydrocarbon cracking generally occurs in fired tubular reactors in the radiant section of the furnace. In a convection section, a hydrocarbon stream may be preheated by heat exchange with flue gas from the furnace burners, and further heated using steam to raise the temperature to incipient cracking temperatures, typically 500-680° C. depending on the feedstock.
After preheating, the feed stream enters the radiant section of the furnace in tubes referred to herein as radiant coils. It should be understood that the method described and claimed can be performed in ethylene cracking furnaces having any type of radiant coils. In the radiant coils, the hydrocarbon stream is heated under controlled residence time, temperature and pressure, typically to temperatures in the range of about 780-895° C. for a short time period. The hydrocarbons in the feed stream are cracked into smaller molecules, including ethylene and other olefins. The cracked products are then separated into the desired products using various separation or chemical-treatment steps.
Various byproducts are formed during the cracking process. Among the byproducts formed is coke, which can deposit on the surfaces of the tubes in the furnace. Coking of the radiant coils reduces heat transfer and the efficiency of the cracking process as well as increasing the coil pressure drop. Therefore, periodically, a limit is reached and decoking of the furnace coils is required.
Decoking of ethylene furnaces is typically conducted every 20 to 70 days. Because the decoking process is generally difficult to monitor, prior decoking procedures are accomplished by ramping air and steam flows at historically acceptable values based upon experience. Using these procedures, it can be difficult to control the coke burn rate. It is also difficult to detect conditions that require a slower more conservative decoke procedure (slower ramping of air rate). This can result in damage to the radiant coils or an undesirably slow decoking, increasing furnace down time and reducing production.
For example, to avoid damage to the radiant coils, some more conservative decoking procedures utilize low air and steam flow rates and flow ramping rates at the beginning of the decoking procedure to avoid fast coke burn. These more conservative procedures can lead to increased down time and lost production. On the other hand, air and steam flow rates and flow ramp rates that are too fast can cause coil erosion or localized fast burning, which can damage the radiant coils.
When air is first introduced to the furnace to start the burning of the coke, overheating of the radiant coils can occur causing a reduction in coil life. Control of the initial air introduction step is difficult because no direct measurement of the coke burning rate is available. To avoid coil damage, this step generally is performed very slowly, which can unnecessarily extend the time for the decoking process.
One effort to address this problem involves the use of effluent analyzers to monitor CO2 formation in the coke burning process. These analyzers generally do not work well at the start of the decoking process due to the relatively small amounts of CO2 present. In addition, the CO2 analysis can be difficult to interpret because it is actually a measure of the percentage of air that is consumed rather than the burn rate of the coke.
Coke spalling prior to decoking is also a concern. Coke can spall from coils due to process upsets immediately prior to decoking and collect in the radiant coils. This material burns very easily, and, as a result, areas of the tubes can be overheated. Coke spalling can be difficult to detect by the methods currently used, which are typically either visual inspection or by measuring coil pressure drop.
Accordingly, it would be desirable to have a method for decoking an ethylene furnace that allowed improved control to reduce the time for the decoking process and to avoid or reduce damage to radiant coils.